The invention relates to a method of chemically polishing medical implants made of titanium or an alloy of titanium to produce a smooth, matte finished surface.
Chemically pure titanium and Ti-6Al-4V alloy are extremely biocompatible materials which are used for a variety of medical applications including implants. When an implant fabricated from such a metal is implanted in the human body, the host tissue surrounding the implant fixes itself to the implant in direct apposition to the implant, in contrast with other common implant materials to which fibrous tissue may tend to encapsulate the implant. Thus titanium and alloys thereof are particularly useful for implants in which the former direct apposition type of fixation is preferable to the later surrounding type of fixation.
If a titanium implant surface is rough, tissue will attach itself mechanically at the implant-tissue interface. Although for a permanent implant, this type of fixation may be appropriate in that it is a predictable stabilizing mechanism, for a semi-permanent (long-term temporary) implant, such permanent fixation is undesirable. When the implant is to be removed, it may be necessary to cut the implant from the surrounding tissue thus creating trauma to the surrounding tissue. Examples of implants intended for temporary or semi-permanent implantation include ventilation tubes, e.g. a vent tube for the tympanic membrane, percutaneous devices for drug infusion, and other similar devices. The invention is particularly suitable for implants having intricate designs and/or of small size which are difficult to polish by conventional metal polishing techniques and for implants designed to create a passageway through surrounding tissue, where tissue ingrowth may clog the passageway decreasing its effectiveness for ventilation and/or fluid drainage.
A further problem with rough or sharp edges on implants is that they may cause an undesirable inflammatory response by surrounding body tissue. A smooth surface is also required for articulating implants, or implants that come in direct contact with blood. For these reasons, among others, such implants should be polished in some manner after they are fabricated.
In the past, metal implants have been finished by a variety of techniques including hand polishing, glass beading, vapor blasting, and electrolytic polishing. Although acceptable for many applications, it is difficult to effectively polish all surfaces and crevices of miniature implants, or implants having intricate designs and/or internal passageways. Mechanical polishing and electrolytic polishing produce a finished product having a bright, light-reflective surface. This type of surface is undesirable because bright light used for inserting the implant can cause considerable glare which interferes with the physician's ability to see the exact location of the implant. Thus it would be desirable to have a polishing technique which reduces glare and is more effective in polishing internal passageways as well as external surfaces of multi-faceted implants and instruments.